Radio receiver, radio communication system, radio communication method, and program

ABSTRACT

A radio receiver including a sampling unit, a provider, an arithmetic operation unit, an estimator, and a converter. The sampling unit samples a baseband signal transmitted from the radio transmitter, at a fractional multiple of a symbol rate, and generates fractional-multiple-sampling data. The provider provides reference data in which the known symbol sequence arranged in a frame by the radio transmitter is interpolated at a rate of the fractional multiple. The arithmetic operation unit performs an arithmetic operation for evaluation data in which the degree of consistency in waveform between the fractional-multiple-sampling data and the reference data is evaluated. The estimator estimates a reference timing from a shift amount at which the evaluation data shows the maximum degree of consistency in waveform. The converter converts the fractional-multiple-sampling data by using the reference timing as a reference thereby recovering the data having the symbol rate.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of co-pending application Ser. No.12/947,974 filed on Nov. 17, 2010, incorporated herein in its entirety,which also claims priority under 35 U.S.C. 119 from Japanese Application2009-269606, filed Nov. 27, 2009, the entire contents of which are alsoincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radio communication technology, andmore specifically, relates to a radio receiver, a radio communicationsystem, a radio communication method, and a program which achievesynchronization in high-speed radio communications.

2. Description of Related Art

Recently, communication rates in information communications have beenincreasing. Higher communication rates have also been increasinglydemanded in radio communications in order to achieve real-timetransmission/reception of rich content, such as a moving images andseamless connections to wired communications. In informationcommunications, it is indispensable to synchronize signals between atransmitting side and a receiving side in order to correctly achievedata transmission/reception. However, in radio communications, a clocksignal is not transmitted independently on the transmitting side, and asynchronization clock is generated on the receiving side. Thus, phasesynchronization between the transmitting side and the receiving side isa challenging task, with the overall performance of a radiocommunication system depending on the quality of the synchronization.

A known technique for data synchronization in radio communications is touse a known pattern sequence. In this technique, a transmitter transmitsa signal of a known pattern sequence to a receiver, the known patternsequence being arranged in the preamble or the header of a frame. Thereceiver calculates a correlation between the received signal and asignal of the known pattern sequence or performs an arithmetic operationfor pattern matching therebetween, and thereby detects the known patternsequence in the received signal. In this way, symbol synchronizationbetween the transmitter and the receiver is achieved.

For example, Japanese Patent Application Publication No. 2001-103044(Patent Literature 1) discloses a symbol synchronization method in whicha receiver oversamples a received signal at a rate of N times persymbol, and then calculates a correlation between the oversampled signaland a known symbol sequence. In the symbol synchronization method, acertain sampling point is selected and used as a symbol timing.Specifically, the certain sampling point is a sampling point existingforemost among sampling points which are near a sampling point havingthe maximum amplitude of a calculated correlation function and whichconcurrently have amplitudes equal to or higher than a predeterminedthreshold.

Japanese Patent Application Publication No. 2003-218967 (PatentLiterature 2) discloses a timing synchronization method in which asynchronization timing point is obtained by: oversampling a receivedframe including multiple synchronization symbols; and then bycalculating a correlation between the multiple digitized synchronizationsymbols and a reference symbol for timing synchronization which has beenset in advance.

Japanese Patent Application Publication No. 2003-234791 (PatentLiterature 3) discloses a detection method of finding a symbol timing byperforming an arithmetic operation for a delay detection of an inputbaseband signal oversampled by N times.

Japanese Patent Application Publication No. 2007-181016 (PatentLiterature 4) discloses a configuration of sampling signals at afrequency which is an integer multiple of a symbol modulation rate.

As described above, in each of the synchronization techniques using aknown pattern sequence, a signal is sampled at a higher rate than asymbol rate, and then a known pattern sequence is detected from theoversampled data. Normally, as disclosed in the aforementioned PatentLiteratures 1 to 4, the signal is oversampled at a rate which is aninteger multiple of a symbol rate, more typically at a rate 2^(N) times(N is an integer) as high as the symbol rate. Therefore, the knownpattern sequence is formed of an integer number of bits. Thus, whichevertechnique of the correlation calculation or pattern matching is used,detection is generally difficult unless the oversampling rate is set asan integer multiple to correspond to the integer number of bits.

In a region in which a relatively low symbol rate is used, theoversampling at a rate of an integer multiple, such as 8, 16, 100, or128 times, can be performed readily even in radio communications.However, in a region in which a data rate exceeding 1 Gbps is used, thebandwidth of the signal and the bandwidth of the circuit are close toeach other. Thus oversampling at a rate of an integer multiple isdifficult.

As an analog to digital converter (ADC) used for oversampling, evenhigh-end ADC's currently available on the market have a performance ofapproximately 4 Gsps. Accordingly, when a data rate of 3 Gbps isassumed, a commercially available ADC cannot achieve oversampling ateven two times, which is the smallest integer multiple. There do existADC's for measuring equipment capable of a performance of up to 50 Gsps,however, from a viewpoint of cost, they are prohibitively expensive fora consumer communication apparatus. In summation, with the increasingrate of data in recent radio communications, the oversampling at aninteger multiple of a symbol rate has become difficult, and thussynchronization has become very difficult.

As described above, in the aforementioned conventional techniquesdisclosed in Patent Literatures 1 to 4, oversampling at an integermultiple is performed in one symbol frequency, and a circuit thereof isconfigured also based on the integer multiple. In a case where any ofthe conventional techniques disclosed in Patent Literatures 1 to 4 isapplied to achieve radio communications at a data rate of 3 Gbps byusing an ADC having a sampling rate of approximately 4 Gsps, therearises a need for an extra conversion process in which upsampling orinterpolation is performed before calculating a correlation value, andthen a signal having a rate of an integer multiple is generated. Thiscauses an additional error due to the upsampling or the interpolation.In addition, when the correlation calculation or the arithmeticoperation of pattern matching is performed, oversampling at a rate ofeven two times, which is the smallest integer multiple, requires aregister two times as long as the known pattern sequence. Consequently,this leads to a largely configured circuit and an increase of powerconsumption.

BRIEF SUMMARY OF THE INVENTION

To overcome these deficiencies, the present invention provides a radioreceiver configured to communicate with a radio transmitter whichtransmits data with a known symbol sequence arranged in a frame, theradio receiver including: a sampling unit for sampling a baseband signaltransmitted from the radio transmitter at a fractional multiple of asymbol rate, wherein fractional-multiple-sampling data is generated, aprovider for providing reference data in which the known symbol sequenceis interpolated at a rate of the fractional multiple, wherein theinterpolation reflects an overall filter characteristic of the radiotransmitter and the radio receiver, an arithmetic operation unit forperforming an arithmetic operation for evaluation data in which thedegree of consistency in waveform between thefractional-multiple-sampling data and the reference data is evaluated,an estimator for estimating a reference timing from a shift amount atwhich the evaluation data shows a maximum degree of consistency inwaveform, and a converter for converting thefractional-multiple-sampling data by using the reference timing as areference wherein the frame having the symbol rate is recovered.

According to another aspect of the invention, the present inventionprovides a radio communication system including a radio transmitterwhich transmits data with a known symbol sequence arranged in a frameand a radio receiver which communicates with the radio transmitter, theradio receiver including: a sampling unit for sampling a baseband signaltransmitted from the radio transmitter at a fractional multiple of asymbol rate, wherein fractional-multiple-sampling data is generated, aprovider for providing reference data in which the known symbol sequenceis interpolated at a rate of the fractional multiple, wherein theinterpolation reflects an overall filter characteristic of the radiotransmitter and the radio receiver, an arithmetic operation unit forperforming an arithmetic operation for evaluation data in which thedegree of consistency in waveform between thefractional-multiple-sampling data and the reference data is evaluated,an estimator for estimating a reference timing from a shift amount atwhich the evaluation data shows a maximum degree of consistency inwaveform, and a converter for converting thefractional-multiple-sampling data by using the reference timing as areference wherein the frame having the symbol rate is recovered.

According to yet another aspect of the invention, the invention providesa method which is executed by a radio receiver configured to communicatewith a radio transmitter which transmits data with a known symbolsequence arranged in a frame, the method including the steps of samplinga baseband signal transmitted from the radio transmitter at a fractionalmultiple of a symbol rate, wherein fractional-multiple-sampling data isgenerated, performing an arithmetic operation for evaluation data inwhich the degree of consistency in waveform between thefractional-multiple-sampling data and reference data is evaluated, thereference data being obtained by interpolating the known symbol sequenceat the rate of the fractional multiple, wherein the interpolationreflects an overall filter characteristic of the radio transmitter andthe radio receiver, estimating a reference timing from a shift amount atwhich the evaluation data shows a maximum degree of consistency inwaveform, and converting the fractional-multiple-sampling data by usingthe reference timing as a reference wherein the frame having the symbolrate is recovered.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a radio communication systemaccording to an embodiment of the present invention.

FIG. 2 is a functional block diagram of a radio receiver of thisembodiment.

FIG. 3 is a diagram explaining a principle of symbol synchronization ofthis embodiment.

FIGS. 4A to 4D are graphs explaining a principle for obtaining areference timing from reference data and fractional-multiple-samplingdata of this embodiment.

FIG. 5 is a diagram explaining a principle for recovering data having asymbol rate from fractional-multiple-sampling data of this embodiment.

FIG. 6 is a diagram explaining in detail a principle for recovering datahaving a symbol rate from fractional-multiple-sampling data of thisembodiment.

FIG. 7 is a graph showing a time response of a raised-cosine filterhaving a roll-off ratio β of 0.25.

FIG. 8 is a graph showing a waveform of an input baseband signal whichis observed on the ADC input side, the input baseband signal obtained bylimiting a band of a transmitter-side baseband signal according to araised-cosine filter characteristic in which the roll-off ratio β is0.25.

FIG. 9 is a graph showing a waveform of sampling data when an inputbaseband signal is sampled at a rate of 5/4 times.

FIG. 10 is a graph showing correlation data calculated based on thefractional-multiple-sampling data and the reference data.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described by using specific embodiments.However, the present invention is not limited only to the embodimentsthat are described.

FIG. 1 is a schematic diagram showing a radio communication system 100according to an embodiment of the present invention. The radiocommunication system 100 of the embodiment includes a radio transmitter120 and a radio receiver 130. The radio transmitter 120 and the radioreceiver 130 establish radio communication using an electromagnetic waveof a frequency band of, for example, several tens of GHz, and achieve adata communication rate of several Gbps.

Generally, in order to perform data communication correctly,synchronization between a transmitter and a receiver is required. Theradio transmitter 120 and the radio receiver 130 of this embodimentachieve symbol synchronization by using a known symbol sequence definedcommonly between the radio transmitter 120 and the radio receiver 130.The radio transmitter 120 arranges data of a known symbol sequence in apreamble 152 in a frame 150, and then transmits the data to the radioreceiver 130. On the other hand, the radio receiver 130 of thisembodiment checks the received data with reference data for detectingthe known symbol sequence (details will be described later), and therebydetects the known symbol sequence in a transmission data signal from theradio transmitter 120. The radio receiver 130 estimates a referencetiming of the known symbol sequence, thereby recovers the frame 150including a payload 154 which is substantial data. Thereby, the radioreceiver 130 performs synchronization of symbol timing with sufficientaccuracy and thus correct data transmission is achieved.

Any sequence can be used as the aforementioned known symbol sequence inthis embodiment using a correlation calculation, as long as the sequencehas a high autocorrelation. Examples thereof include a maximal-lengthsequence (M sequence), a Gold sequence, a Golay sequence, and the like,which are pseudo-random number sequences. Note that “having a highautocorrelation” means “having a characteristic in which a single peakis observed in an autocorrelation function,” preferably “having acharacteristic in which side lobes gradually approximate substantiallyzero.” The high autocorrelation of the known symbol sequence enablesfavorable estimations of the start position of the known symbol sequenceand a sampling phase.

In an example shown in FIG. 1, a laptop personal computer 110, forexample, is connected to the radio transmitter 120, while a display 140,for example, is connected to the radio receiver 130. For example, thecontent of the data, such as, a moving image loaded on the payload 154of the frame 150, is transferred from the laptop personal computer 110via the radio transmitter 120 and the radio receiver 130, and then isdisplayed on a screen of the display 140.

Herein below, symbol synchronization using reference data is describedin more detail. FIG. 2 shows a functional block diagram of the radioreceiver 130 of this embodiment. A functional block 200 of the radioreceiver 130 includes an analog to digital converter (ADC) 210, areference data provider 220, a correlation calculator 230, a peakestimator 240, and a converter 250. Note that the functional block 200shown in FIG. 2 shows only a functional configuration related to thesymbol synchronization, and other functional elements are omitted, suchas an antenna, a radio frequency (RF) signal processor which processesradio wave signals, as well as processors for a media access control(MAC) layer and the subsequent layers, where recovered data is processedwith predetermined protocols.

The ADC 210 receives a baseband signal (I phase/Q phase) transmittedfrom the radio transmitter 120 via the antenna and RF signal processor(not illustrated). The baseband signal to be inputted into the ADC 210is an analog signal which is obtained by limiting a band of atransmitted baseband signal of a modulation signal having apredetermined symbol rate due to a filter characteristic of the radiotransmitter 120 and the radio receiver 130. Hereinafter, the basebandsignal inputted to the ADC 210 is referred to as an input basebandsignal.

The ADC 210 oversamples the input baseband signal at a rate which is afractional multiple (q/p times, where p and q are prime positiveintegers) of the above-described symbol rate, and then generatesfractional-multiple-sampling data. The generatedfractional-multiple-sampling data is outputted from the ADC 210, andthen inputted into the correlation calculator 230 and the converter 250.Note that the ADC 210 provides a sampling unit of this embodiment.

The correlation calculator 230 performs a correlation calculation onreference data inputted from the reference data provider 220 by directlyusing the fractional-multiple-sampling data inputted from the ADC 210,generates correlation data which is a function of a shift amount of acorrelation value, and outputs the correlation data to the peakestimator 240. The correlation calculator 230 provides an arithmeticoperation unit of this embodiment, and the correlation data providesevaluation data in which the degree of consistency in waveform betweenthe reference data and the fractional-multiple-sampling data isevaluated. Although the details of the reference data will be describedlater, the reference data is data for detecting the known symbolsequence arranged in the preamble 152 of the frame 150 from thetransmitting side, by performing the correlation calculation directlywith the fractional-multiple-sampling data, and is provided by thereference data provider 220.

From a versatility viewpoint, the reference data provider 220 can beconfigured as follows. Specifically, the reference data provider 220calculates the reference data beforehand from the known symbol sequence,stores the data as a table in a memory such as a read only memory (ROM),reads the data from the memory upon receipt of the baseband signal, andthen provides the data as a correlation coefficient to the correlationcalculator 230. In this case, even a change to a different known symbolsequence can be made by rewriting new reference data in the memory.Alternatively, when the known symbol sequence is fixed or semi-fixed,the reference data is hard-wired and internally configured to beimplemented in the correlation calculator 230. This can improve theperformance and reduce the circuit size. The hard-wired implementationmight be preferable, depending on the oversampling rate and the lengthof the known symbol sequence. Note that the reference data provider 220provides a provider of this embodiment.

The peak estimator 240 estimates a reference timing of the known symbolsequence from the inputted correlation data, and outputs the referencetiming to the converter 250 as parameters such as a start position and asampling phase. The converter 250 performs interpolation processing onthe fractional-multiple-sampling data inputted from the ADC 210 inaccordance with the reference timing (the start position and thesampling phase of the known symbol sequence) estimated by the peakestimator 240, recovers data which is synchronized in correct phases andhas an original symbol rate, and then outputs the data to a MACprocessor or the like provided for subsequent processing. The peakestimator 240 and the converter 250 provide an estimator and a converterof this embodiment, respectively.

Hereinbelow, an operation mechanism of the symbol synchronization usingthe reference data and the fractional-multiple-sampling data will bedescribed in more detail by referring to FIGS. 3 to 6. FIG. 3 is adiagram explaining a principle of the symbol synchronization of thisembodiment. Note that for the sake of convenience, the description isgiven in this embodiment by taking, as an example of a modulationmethod, a binary phase shift keying (BPSK) modulation which is a binarymodulation. In addition, in the description below, a Golay sequence of128 bits “3663FAAFFA50369CC99CFAAF05AF369C” is used as a known symbolsequence. However, the known symbol sequence and the length thereof arenot limited to the exemplary ones, and can be set in consideration ofthe accuracy of detection and the overhead of communication.

As to a modulation method, this embodiment is also applicable to variousmodulation methods, for example, amplitude shift keying (ASK) andfrequency shift keying (FSK), multilevel modulation methods such asmultiple phase shift keying (MPSK), multiple frequency shift keying(MFSK), multiple amplitude shift keying (MASK), and quadrature amplitudemodulation (QAM), and others. When the multilevel modulation method isadopted, what is needed is to configure an appropriate preamble so thata symbol sequence having a high autocorrelation can be obtained afterperforming mapping corresponding to the modulation method. Here, thesymbol sequence is expressed by a complex function formed of an I phaseand a Q phase.

The description below is given of the embodiment on the assumption that5/4 times is selected as the oversampling rate. The fractional-multiplerate is not particularly limited, but preferably takes on a value in arange of 1<(q/p)<2 from a viewpoint of effective utilization of theprinciple of the embodiment of the present invention and from aviewpoint of power consumption, and more preferably 1.2≦q/p from aviewpoint of a detection accuracy. In general, the larger an integer qis, the more preferable. Meanwhile, an integer p preferably takes on asmall value in consideration of the arithmetic operational efficiency,power consumption, and complexity of an arithmetic operation inrecovering the data having the symbol rate from thefractional-multiple-sampling data. To be more specific, fraction valuesof 3/2, 4/3, 5/3, 5/4, 7/4, 6/5, 7/5, 8/5, and 9/5 can be cited asexamples of a favorable parameter, and 3/2, 4/3, and 5/3 are morepreferable among the above.

Herein below, a description is given of how a baseband signal(transmitter-side baseband signal) having a symbol frequency T andcorresponding to a known symbol sequence is observed on the radioreceiver 130 side when the baseband signal is transmitted from the radiotransmitter 120 to the radio receiver 130. In this case, an inputbaseband signal observed on the input side, or on the ADC 210 of theradio receiver 130 (a receiver-side input baseband signal), is a signalobtained by limiting the band of the transmitter-side baseband signaldue to the filter characteristic of a radio transmitter and a radioreceiver as the whole (hereinafter, referred to as an overall filtercharacteristic). For example, when the overall filter characteristic isexpressed by a raised cosine filter, a time response h(t) can beexpressed by the following equation (1). The time response is reflectedin the receiver-side input baseband signal.

$\begin{matrix}{{h(t)} = {\frac{\sin\left( \frac{\pi\; t}{T} \right)}{\frac{\pi\; t}{T}}\frac{\cos\left( \frac{{\pi\beta}\; t}{T} \right)}{1 - \frac{4\beta^{2}t^{2}}{T^{2}}}}} & (1)\end{matrix}$

In the equation above, T denotes a symbol rate; β, a roll off ratio; andt, time. FIG. 7 is a graph showing a time response of a raised cosinefilter having the roll off ratio β of 0.25. Note that the symbol rate Tis 1 in FIG. 7.

On the radio receiver 130 side, the ADC 210 oversamples, at a rate ofq/p times, the receiver-side input baseband signal obtained by limitingthe band of the transmitter-side baseband signal, and then generatesfractional-multiple-sampling data (receiver-sidefractional-multiple-sampling data). FIG. 8 is a graph showing a waveformof the input baseband signal which is observed on the ADC input side.The input baseband signal is obtained by limiting the band of thetransmitter-side baseband signal due to the raised cosine filtercharacteristic which uses the exemplified known symbol sequence and inwhich the roll off ratio β is 0.25. FIG. 9 is a graph showing a waveformof sampling data in a case of sampling the input baseband signal at therate of 5/4 times.

In order to detect the known symbol sequence in the actualfractional-multiple-sampling data, certain data is calculated as thereference data in this embodiment, the certain data corresponding to awaveform obtained in the case where the band of the baseband signal ofthe predetermined known symbol sequence is band-limited due to theoverall filter characteristic and the baseband signal is sampled at thefractional-multiple rate. Generally, the overall filter characteristicis known, since the filter characteristic is designed by dividing thefunctions to the radio transmitter side and the radio receiver side sothat a desirable characteristic overall can be obtained.

In this embodiment, the description is given by taking as an example acase where the overall filter characteristic is designed as the raisedcosine filter. The raised cosine filter, however, is merely an example.As long as the waveform of the overall filter characteristic is known,the filter shape of the characteristic is not limited. Any filter usedin radio communication technology is conceivable, and for example, aBessel filter, Chebyshev filer, a Butterworth filter, or a Gaussianfilter can be used besides the raised cosine filter. A filteringfunction or an approximation thereof according to the filtercharacteristic can be defined.

When the overall filter characteristic is designed as the raised cosinefilter, a filtering function shown in the above-described equation (1)expressing the raised cosine filter is used as an interpolation formula,and thereby the reference data is generated by interpolating the knownsymbol sequence at the rate of q/p times in the enlarged manner. To bemore specific, a table is created by interpolating the signal of theknown symbol sequence by an integer multiple (q times) by use of thefiltering function, and then the signal is thinned out at intervals ofthe number of an integer (every p) in the table. Thereby, the referencedata interpolated at the rate of q/p times in the enlarged manner can beobtained. The length of the reference data is approximately the knownsymbol sequence multiplied by the fractional multiple.

The above-described equation (1) can be directly used as aninterpolation formula actually used for the enlarged interpolation.Alternatively, an approximation such as a polynomial approximation orthe like of the above-described equation (1) may also be used. Thepolynomial approximation can secure sufficient accuracy even in athird-order polynomial, and can also reduce the arithmetic operationalprocessing in the hard-wired implementation. Considering that theinterpolation formula is used also in recovering the data having thesymbol rate from the fractional-multiple-sampling data, it is preferableto use an interpolation formula using the polynomial approximation.

Consider the case of the polynomial approximation where an approximationfunction of the filtering function is F(t)=at³+bt²+ct+d. In this case,when a section between two central points in four specific points isinterpolated, such coefficients a, b, c and d that allow theapproximation function to always pass through the four specific pointsare obtained, and an appropriate interpolation ratio t is set. Thereby,an interpolation value can be obtained.

FIGS. 4A to 4D are graphs explaining the principle for obtaining thereference timing from the reference data and thefractional-multiple-sampling data of this embodiment. FIG. 4Aschematically shows the fractional-multiple-sampling data inputted fromthe ADC 210 to the correlation calculator 230. FIG. 4B schematicallyshows the reference data inputted from the reference data provider 220to the correlation calculator 230. The correlation calculator 230calculates a crosscorrelation value R (τ) between the reference data andthe fractional-multiple-sampling data in accordance with the followingequation (2), and then outputs the crosscorrelation value R (τ) to thepeak estimator 240.

$\begin{matrix}{{R(\tau)} = {\sum\limits_{k = 0}^{L}{{{ref}(k)} \cdot {{smpl}\left( {k + \tau} \right)}}}} & (2)\end{matrix}$

In the above-described equation (2), L denotes the length of the knownsymbol sequence obtained by multiplying the known symbol sequence by afraction; ref(t), a t-th(t=0 to L) value of the reference data; smpl(t),a value of a t-th(t=0 to L) sampling point of thefractional-multiple-sampling data; and τ, a shift amount.

FIG. 4C schematically shows the correlation data calculated from: thereference data calculated from the known symbol sequence; and theobserved fractional-multiple-sampling data. As shown in FIG. 4C, whenthe fractional-multiple-sampling data includes the known symbolsequence, a peak is observed in the correlation value, the peak having ashift amount approximately corresponding to the beginning of the knownsymbol sequence. Since a timing at which the peak is observedapproximately corresponds to the beginning of the known symbol sequence,data can be recovered by using this timing as a reference. The peak canbe determined, for example, as a point exceeding a threshold set inadvance.

FIG. 10 is a graph showing correlation data calculated from thefractional-multiple-sampling data shown in FIG. 9 and the referencedata. As shown in FIG. 10, a correlation value calculated from thefractional-multiple-sampling data oversampled at the fractional multipleand the reference data interpolated by the fractional multiple in theenlarged manner also shows a high autocorrelation with the known symbolsequence. Thus, it can be understood that the reference timing of theknown symbol sequence can be preferably detected.

In almost any case, circuits on the radio transmitter 120 side and theradio receiver 130 generally have phases shifted from each other. Forthis reason, the sampling point of the data sampled at afractional-multiple rate does not necessarily match an original peakposition. FIG. 4D is a graph obtained by enlarging a portion near thepeak in the correlation data. As shown in FIG. 4D, it is conceivablethat the original peak is located between sampling points. Hence, inthis embodiment, the peak estimator 240 uses correlation values ofmultiple sampling points near the peak of the correlation data toestimate a shift amount of the original peak.

Generally, a waveform of a correlation value is a band-limited pulsewaveform. Thus, by using a few points (for example, three points) aroundthe sampling point at which the correlation data shows the maximumcorrelation value, a shift amount t_(max) of an original peak can beestimated by use of the above-described interpolation formula.Meanwhile, when the waveform of the time response of the filter isviewed, a portion near the peak in the band-limited pulse waveform canbe preferably approximated by a quadratic function. Thus, based on threepoints or more around the sampling point at which the correlation datashows the maximum correlation value, the shift amount t_(max) of anoriginal peak can be estimated by using the approximation. The estimatedshift amount t_(max) shows the start position and the sampling phase ofthe known symbol sequence.

In the approximation of the quadratic function, the approximationfunction is set as F(t)=at²+bt+c. Then, coefficients a, b and c areobtained, the approximation function can pass through the three pointsnear the peak, and thereby t_(max) can be estimated as an extreme value(=b/2a) of the coefficients. Alternatively, t_(max) can be estimated bycurve fitting from a coefficient having the smallest square error. Sincethe thus obtained t_(max) determines an interpolation ratio for theinterpolation, correct data can be recovered from the data points aroundthe sampling point.

When the reference timing is estimated, the converter 250 converts thefractional-multiple-sampling data into data having a symbol rate inaccordance with the reference timing received from the peak estimator240, and thus recovers the entire frame. The recovering act uses theinterpolation formula used in generating the reference data. The integerp of q/p is a rational number since the value of the fractional multipleused for oversampling is preferably a small value in consideration ofthe complexity of the arithmetic operation, the efficiency of thearithmetic operation, and the power consumption for the recovering act.Since all the received frames are inputted to the converter 250, theoverall performance can be improved by making the power consumption moreefficient and the arithmetic operation of the recovering processing inthe last stage using the interpolation formula.

FIG. 5 is a diagram explaining a principle in which the data having thesymbol rate is recovered from the fractional-multiple-sampling data ofthis embodiment. As shown in FIG. 5, original data transmitted from theradio transmitter 120 is data having the symbol rate frequency T. Thereference data is prepared so that the start position of the knownsymbol sequence included in the original data can be detected. Theactually measured fractional-multiple-sampling data generally has ashifted phase relative to the original data, and is measured in a statewhere the start position of the data is unclear. However, by calculatingthe correlation with the reference data, the start position of the knownsymbol sequence and the sampling phase difference are evaluated. Then,the data having the original symbol rate is recovered from thefractional-multiple-sampling data in accordance with the evaluatedreference timing.

FIG. 6 is a diagram explaining the details of the principle forrecovering the data having the symbol rate from thefractional-multiple-sampling data of this embodiment. In the recoveringprocessing, the fractional-multiple-sampling data oversampled by q/ptimes is firstly interpolated by the integer multiple p by using theinterpolation formula, so that data interpolated by p times isgenerated. The interpolation formula is the same as the aforementionedone used in generating the reference data.

As shown in FIG. 6, there are q cases of how the data interpolated by ptimes is thinned out every q which is an integer to have the originalsymbol rate. Preferably, a sequence having a sampling point closest to apeak position of the shift amount t_(max) is selected from among the qtypes of sequences, and then the selected sequence is interpolated at aninterpolation ratio obtained from the shift amount t_(max) by using theabove-described interpolation formula. Thereby, the data having thesymbol rate can be recovered. Thereafter, by using the interpolationratio described above, similar processing is performed on thefractional-multiple-sampling data corresponding to the entire frameincluding the payload 154 following the preamble 152. Thereby, a bitsequence of the entire frame can be recovered. As described above, whenthe oversampling by q/p times is adopted, since the sequence having theoriginal symbol rate is recovered by interpolating the data by p times,a smaller integer p results in a smaller number of cases and thussmaller load on the arithmetic operation.

According to the aforementioned embodiment of the present invention, theknown symbol sequence is detected after the degree of the matching ofthe waveform between the reference data and the actual measurement isevaluated, the reference data obtained by interpolating the known symbolsequence in the enlarged manner at the fractional-multiple rate withrespect to the symbol rate by reflecting the overall filtercharacteristic of the transmitter and the receiver. Thereby, instead ofoversampling at an integer multiple, the reference timing in datatransmitted from the transmitter can be preferably identified and thussymbol synchronization can be achieved.

In addition, according to the aforementioned embodiment, simplyperforming a correlation calculation on the reference data and thesampling data which have the fractional-multiple rate substantiallyleads to a result including an overall filter characteristic of thetransmitter and the receiver and an oversampling characteristic. Thiscan avoid such extra processing that causes an additional error, andthus achieves highly accurate synchronization. Furthermore, the datalength of the reference data used for the correlation calculation can bemade shorter than in a conventional case in which a register having adata length of an integer multiple needs to be secured. This canpreferably avoid a larger circuit configuration and increase of powerconsumption.

In the aforementioned embodiment, the description has been given inwhich the value of crosscorrelation between the reference data and thefractional-multiple-sampling data is calculated and the reference timingis obtained from the peak position of the crosscorrelation value.However, a pattern matching method can be adopted instead of thecorrelation calculation in another embodiment. In this case, a patternmatching processor is configured instead of the correlation calculator230. The pattern matching processor calculates an evaluation valueindicating the degree of the matching of the waveform between thereference data and the fractional-multiple-sampling data. The peakestimator 240 estimates a timing at which the matching becomes thelargest from the evaluation data outputted from the pattern matchingprocessor.

The radio receiver according to the embodiment of the present inventionis not restricted by the specifications of the radio transmitter whichis the communication counterpart. As long as the filter characteristicof the radio transmitter is known, the overall filter characteristic canbe obtained together with the filter characteristic of the receiver.Thus, the reference data can be provided according to the thus obtainedoverall filter characteristic.

As described above, the embodiment of the present invention can providea radio receiver, a radio communication system, a radio communicationmethod, and a program which can achieve symbol synchronization withsufficient accuracy even in high-speed radio communications in which aband of a signal and a band of a circuit are close to each other, andthus oversampling at a rate of an integer multiple is difficult.

The aforementioned function of the present invention can be implementedas a device-executable program written in a legacy programming languageor an object oriented programming language such as an assembler, C, C++,Java (registered trademark of Sun Microsystems), Java (registeredtrademark of Sun Microsystems) Beans, Java (registered trademark of SunMicrosystems) Applet, Java (registered trademark of Sun Microsystems)Script, Perl, or Ruby. The program can be distributed by being stored inany device-readable recording medium or distributed by being transmittedthrough a network.

Although having been described above by using the specific embodiment,the present invention is not limited to the embodiment, but can bechanged within a range in which those skilled in the art can come upwith by implementing another embodiment, or by adding, changing, oromitting any element of the present invention. Any modes thus madeshould be included within the scope of the present invention, as long asthese modes provide the same operations and advantageous effects asthose of the present invention.

What is claimed is:
 1. A non-transitory computer readable storage mediumcomprising a computer readable program for radio communication executedby a radio receiver configured to communicate with a radio transmitterwhich transmits data with a known symbol sequence arranged in a frame,wherein the computer readable program when executed on a computer causesthe computer to perform the steps of: sampling a baseband signaltransmitted from said radio transmitter at a fractional multiple of asymbol rate, wherein fractional-multiple-sampling data is generated;performing an arithmetic operation for evaluation data in which thedegree of consistency in waveform between saidfractional-multiple-sampling data and reference data is evaluated, saidreference data being obtained by interpolating said known symbolsequence at said rate of said fractional multiple, wherein saidinterpolation reflects an overall filter characteristic of said radiotransmitter and said radio receiver; estimating a reference timing froma shift amount at which said evaluation data shows a maximum degree ofconsistency in waveform; and converting saidfractional-multiple-sampling data by using said reference timing as areference wherein said frame having said symbol rate is recovered. 2.The computer readable storage medium according to claim 1, wherein avalue of said fractional multiple is a rational number larger than 1 butsmaller than
 2. 3. The computer readable storage medium according toclaim 2, further comprising: referring said reference data generatedbeforehand by interpolating said known symbol sequence at said rate ofsaid fractional multiple by using a filtering function which expressessaid filter characteristic.
 4. The computer readable storage mediumaccording to claim 3, further comprising: performing an arithmeticoperation for the evaluation of data between saidfractional-multiple-sampling data and said reference data by a methodselected from the group consisting of a calculation of correlation andpattern matching processing.